Sophisticated industrial processes, such as oil refining, automobile assembly or power generation, require the cooperative execution of numerous interdependent tasks by many different pieces of equipment. The enormous complexity of ensuring proper task sequencing and management, which requires not only procedural logic but constant monitoring of equipment states to organize and distribute operations and detect malfunction, has resulted in the widespread adoption of programmable controllers. These controllers operate elaborate industrial equipment in accordance with a stored control program. When executed, the program causes the controller to examine the state of the controlled machinery by evaluating signals from one or more sensing devices (e.g., temperature or pressure sensors), and to operate the machinery (e.g., by energizing or de-energizing operative components) based on a procedural framework, the sensor signals and, if necessary, more complex processing. The "inputs" to a particular controller can extend beyond the sensed state of the equipment the controller directly operates to include, for example, its environment, the state of related machinery or the state of its controllers.
Control requirements become even more complex when different aspects of the same overall process are assigned to remotely situated equipment. Such configurations often require reliable, high-bandwidth serial communication links to provide the necessary interconnection and handle data transfer among controllers and the sensors relevant to their operation.
Ordinarily, process operation is monitored, at least intermittently, by supervisory personnel by means of one or more central management stations. Each station samples the status of controllers (and their associated sensors) selected by the operator and presents the data in some meaningful format. The management station may or may not be located on the same site as the monitored equipment; frequently, one central station has access to multiple sites (whether or not these perform related processes). Accordingly, communication linkage can be vital even in traditional industrial environments where process equipment is physically proximate, since at least some supervisory personnel may not be.
To facilitate the necessary communication, the controller processors and related computers (such as monitoring stations) are arranged as a computer network. A network, basically, is a collection of interconnected computers that use some consistent protocol to communicate with one another. Typically, the network is organized such that any computer may communicate with any other network computer. The communication protocol provides a mechanism by which messages can be decomposed and routed to a destination computer identified by some form of address. The protocol may place a "header" of routing information on each component of a message that specifies source and destination addresses, and identifies the component to facilitate later reconstruction of the entire message by the destination computer. This approach to data transfer permits the network to rapidly and efficiently handle large communication volumes without reducing transfer speed in order to accommodate long individual messages, or requiring every network computer to process every network message. The degree of routing depends on the size of the network. Each computer of a local network typically examines the header of every message to detect matches to that computer's identifier; multiple-network systems use routing information to first direct message components to the proper network.
Controllers have been interconnected by means of computer networks for some time; see, e.g., U.S. Pat. No. 5,307,463. In typical systems, a monitoring computer, which may be remotely located from any or all of the controllers to which it has access, periodically queries the controllers to obtain data descriptive of the controlled process or machine, or the controller itself. This data is then available for analysis by the monitoring computer. Heretofore, however, the type of information obtainable, on demand, from a controller has been limited, while the interface used to present the information on the monitoring computer is typically crude. The latter condition results from the multiplicity of data types offered by the controller. Were each type of data to be rendered in a format suited to that data, it would be necessary to equip the monitoring computer with multiple interfaces, and configure the operating application to ascertain the type of data before selecting and launching the proper interface. Not only does this scheme impose a substantial real-time support burden on the monitoring computer, but would also require a constantly expanding repertoire of graphical capabilities keyed to new forms of data reported by the controllers. This can be especially cumbersome in highly--even internationally--distributed environments, since the controllers and their functionalities may be programmed by personnel having no contact with those responsible for central monitoring.